The present invention relates generally to light valves and more particularly to the use of a reflective matrix mirror in liquid crystal light valves. The invention further relates to using liquid crystal light valves for converting visible wavelength images to infrared wavelength images.
In the development of infrared imagers or "seekers" for advanced search and tracking systems, it is important to be able to test the infrared imagers with the same complex, dynamic scenes as they will encounter under actual field conditions. It is also important to be able to do so non-destructively to minimize cost and time factors.
Therefore, computerized simulation systems are used for non-destructive infrared imager testing. Various field tests can thus be simulated and projected to the infrared imager for performing parametric testing in the laboratory. The advantages of such simulation systems are that they facilitate mission variation, accurately reproduce test environments, and increase proficiency in implementing improvements.
Computerized simulation systems generate an infrared coded visible wavelength image on a cathode ray tube (CRT) and convert it to a high-power infrared image which is projected onto an infrared imager under test. Both the CRT and the infrared imager are coupled to a computer which generates the visible images and monitors infrared images "seen" by the infrared imager. The heart of the simulator is an image converter which takes the infrared coded visible images on the CRT and converts them to the infrared images sensed by the infrared imager.
Ideally, a simulation system should be capable of providing a complex, fast-changing infrared scene with a high degree of dynamic range and resolution, and likewise be capable of real-time interaction with the computerized image generation system. Existing simulation systems have difficulty in meeting these criteria, primarily because of the prior IR conversion devices which have been used.
These prior devices typically utilize thermally produced radiation from thin-film materials for image conversion. Operation of such devices consists of illuminating the film with a high intensity visible beam containing scenery information from a CRT (or movie projector). The film heats up locally in response to the image intensity of the input visible beam and then emits an infrared wavelength pattern that is related to the spatial information presented by the high intensity visible input beam. With such devices, thermal images are projected directly onto the infrared imagers, and the input visible source is directly coupled to the output infrared image, both of which are disadvantageous.
In addition, the thin film infrared simulation systems suffer from low resolution caused by thermal spread in the image plane; slow thermal response caused by large thermal time constants; and limited dynamic range due to the conflict between higher resolution and the dynamic range of a thermal based simulation system.
Liquid crystal-based "light valves" or spatial light modulators have been proposed for use as infrared image converters due to their ability to modulate an infrared beam by polarization-rotation. An example of such a device is set forth in U.S. Pat. No. 4,114,991, to W. T. Bleha on Sept. 19, 1978. The development of, and theory underlying, light valve technology is illustrated in such patents as U.S. Pat. No. 3,824,002, issued to T. D. Beard on July 16, 1974 and U.S. Pat. No. 4,019,807, issued to D. D. Boswell, et al. on April 26, 19777. The basic design of alternating-current (AC) light valves is shown in the Beard patent. The configuration and operation of the light valve in the hybrid field effect mode, which accomplished the polarization-rotation necessary to effect spatial modulation of an infrared beam is discussed in the Boswell Patent, as well as in U.S. Pat. No. 4,378,955, issued to W. T. Bleha on Apr. 5, 1983. All of these patents are owned by the assignee of the present invention.
Several problems exist with employing existing light valves in the infrared wavelength spectral region which relate to the electro-optical properties of the materials used.
Existing reflection-mode light valves used for visible to infrared conversion, such as that shown in U.S. Pat. No. 4,114,991 pass an infrared beam through a relatively thick silicon photosubstrate. This results in distortion introduced by multiple reflections at the silicon/liquid crystal interface and by internal scattering from crystalline defects and inhomogeneities.
Furthermore, in the prior art light valves, the design of a suitable dielectric mirror for use in the infrared spectral region, generally at 5-10 .mu.m, is often difficult. In order to provide sufficient reflection for long wavelength infrared radiation, the dielectric mirrors are often relatively thick multi-layer structures. This creates mirrors having large electrical impedances with respect to the liquid crystal layers employed. Since the optical conversion efficiency is proportional to the ratio of the mirror impedance to the liquid crystal impedances, it is therefore undesirable to have a very large mirror impedance.
Prior art light valves are also primarily AC devices. It would be beneficial to eliminate the AC requirement for the light valves and operate them in a DC configuration.
Lastly, in a missile testing environment, it is desirable to provide for ever-increasing speed and contrast capabilities as the speed and performance of the missiles increase. The dielectric mirrors in the prior art, because of the limitations in their design, are often not suitable for extremely high-speed imaging.
Accordingly, it is the principal purpose of the present invention to provide a high-speed, high-contrast liquid crystal light valve to perform visible to infrared image conversion.
Another purpose of the present invention is to eliminate distortion at the interface between the silicon substrate and the liquid crystal.
A further purpose of the present invention is to provide a liquid crystal light valve capable of operating in a AC or DC configuration.
Yet another purpose of the present invention is to operate a mirror in a liquid crystal light valve in the infrared region and to achieve both high infrared reflectivity and low series resistance.